DOI 10.1140/epja/i2009-10857-7 Regular Article – Experimental Physics Eur. Phys. J. A 42, 471–475 (2009) T HE EUROPEAN P HYSICAL JOURNAL A Yrast spectroscopy in 49–51 Ti via fusion-evaporation reaction induced by a radioactive beam M. Niikura 1, a , E. Ideguchi 1 , N. Aoi 2 , H. Baba 2 , T. Fukuchi 2 , Y. Ichikawa 2 , H. Iwasaki 3 , T. Kubo 2 , M. Kurokawa 2 , M. Liu 4 , S. Michimasa 1 , T. Ohnishi 2 , T.K. Onishi 3 , S. Ota 1 , S. Shimoura 1 , H. Suzuki 2 , D. Suzuki 3 , Y. Wakabayashi 1 , K. Yoshida 2 , and Y. Zheng 4 1 Center for Nuclear Study, the University of Tokyo, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 2 RIKEN Nishina Center, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 3 Department of Physics, the University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan 4 Institute of Modern Physics, Chinese Academy of Sciences, 509 Nanchang Rd., Lanzhou 730000, China Received: 31 December 2008 / Revised: 24 April 2009 Published online: 20 August 2009 – c  Societ`a Italiana di Fisica / Springer-Verlag 2009 Communicated by C. Signorini Abstract. In-beam γ-ray spectroscopy of high-spin states in 49–51 Ti was performed via the fusion- evaporation reaction using a radioactive beam. By excitation function and γ-γ coincidence analysis, yrast high-spin levels up to I = (21/2 - ), (11 + ), (17/2 - ) in 49–51 Ti were determined. The levels were compared with full-pf -shell model calculation. The level structure indicates the persistency of the N = 28 shell gap at yrast states in 49–51 Ti. PACS. 25.60.Pj Fusion reactions – 29.30.Kv X- and γ-ray spectroscopy – 23.20.Lv γ transitions and level energies – 27.40.+z 39 ≤ A ≤ 58 In neutron-rich A ∼ 50 isotopes, studies of the shell structure are recently gaining much attention from both the theoretical and experimental point of view. One ex- ample is the appearance of N = 32 and/or 34 sub-shell closures by changing the single-particle orbits in this mass region [1–5]. A spectroscopic study of the yrast high-spin states provides important information on the presence of shell gaps, since large jumps in transition energies at high- spin values are often assessed as an indicator of excitations that involve breaking of the core; states with higher an- gular momentum are generated from excitations across a shell gap. In the yrast levels of 50 Ti, a large gap in the exci- tation energy between 6 + and 7 + states is understood as a one-particle one-hole (1p1h) excitation across the N = 28 shell gap [6], and many shell model calculations predict that the N = 28 gap persists in neighboring nuclei of 50 Ti. We have performed the in-beam γ -ray spectroscopy to study the shell structure at high spin in 49–51 Ti via the fusion-evaporation reaction using a secondary 46 Ar beam by the 9 Be( 46 Ar, x n) 55−x Ti reaction. In order to populate high-spin states of neutron-rich Ti isotopes, 49–51 Ti, we have developed a new method using a low-energy neutron-rich radioisotope (RI) beam. The fusion-evaporation reaction is commonly used for γ - a Conference presenter; Present Address: Institut de Physique Nucl´ eaire d’Orsay, IN2P3-CNRS, France; e-mail: niikura@ipno.in2p3.fr ray spectroscopy of high-spin states since a large amount of angular momentum can be brought into the system. However, nuclides produced by the reaction using stable isotopes are limited, in many cases, to the proton-rich side of the β-stability line. The use of a neutron-rich RI beam enables us to populate high-spin states of neutron-rich nu- clei. In order to realize such studies, a development of low-energy RI beams by using energy degraders was per- formed. The experiment was performed at the RIKEN Accelerator Research Facility. A secondary 46 Ar beam was produced by the projectile-fragmentation reaction of a 48 Ca primary beam at 63 MeV/nucleon on a 9 Be tar- get with the thickness of 1.625 mm. The secondary beam was separated by the RIKEN Projectile-fragment Sepa- rator (RIPS) [7]. A curved aluminum degrader with the thickness of 0.6 mm placed at the momentum-dispersive focal plane (F1) of RIPS was used to achieve a clear iso- tope separation and to lower the energy of the beam to ∼ 24 MeV/nucleon. The particle identification of the frag- ments was performed by measuring a time of flight (TOF) and an energy loss (ΔE). The TOF was obtained from the timing information between a plastic scintillator of 0.1 mm thickness placed at the achromatic focus (F2) and two Parallel-Plate Avalanche Counters (PPACs) [8] placed at the final focus (F3). The TOF of the beam was also used for beam energy measurements. A nearly pure (99%) 46 Ar beam was obtained at F2.